Nowadays each wing fuselage connection features a plurality of boreholes for drilling bushings. Due to different curvatures of the wing and fuselage surfaces the bushings and accordingly the boreholes offer different depths. Furthermore the diameter of the borehole, the grip of rivet and the shape of rivet (conical or cylindrical) could differ.
Hence there is some risk that the worker by mistake chooses the wrong drill and thus—drills the wrong borehole.